The present invention relates to optical filters for telescopes, specifically those used for viewing the sun and the moon.
It is common knowledge that direct viewing of the midday sun with the naked eye can be damaging to the eye, and this is particularly true when the sun is viewed through a telescope. Telescopic solar viewing therefore requires the use of filters to block out all but a very small fraction of the light reaching the telescope from the sun. Viewing of a bright moon through a telescope can also be uncomfortable, and moon filters are therefore often employed.
From the point of view of the accuracy of the telescope system, it is best that the filter be positioned near to the eyepiece, or at least after the objective, so that the image will have been formed before any errors resulting from the filter are introduced. As a result, filters are sometimes positioned just inside the eyepiece. However, solar filters are frequently positioned at the front of the telescope so that the interior of the telescope will be shielded from the bulk of the solar radiation. This avoids the internal heating that would otherwise result, but the heating is avoided at the expense of accuracy because any distortion caused by the filter occurs before the focusing and image forming of the telescope.
The object of the present invention is to permit easy conversion from sun viewing to moon viewing and back again. A further object is to accomplish this in an assembly that fulfills the purpose of avoiding solar heating of the telescope interior while minimizing the effect of any distortions introduced by the filter.